This invention relates to electrochromic devices for the control and modulation of light transmission, absorption and reflection, and more particularly to the formation of chemically reduced electrode layers for such devices.
Electrochromic devices employ materials with alterable optical properties, for example reversibly, in response to an applied potential. The process involves the simultaneous insertion or extraction of electrons and charge compensating ions. Electrochromic materials and devices are used in, for example, display devices, variable reflectance mirrors, and in the future as windows with controlled light transmission.
In general, electrochromic devices have a composite structure for which the transmittance of light can be varied in response to an applied electrical potential. The composite includes a variably transmissive electrochromic layer, i.e. electrode, that can be normally colorless, but becomes colored when chemically reduced by the insertion of electrons and charge compensating ions. The coloration involves absorption, reflectance or a combination of both.
The composite device also can include a counterelectrode, which can serve as a complementary electrochromic layer. The counterelectrode is either optically passive when chemically oxidized or reduced, i.e. is non-electrochromic, or is colored when oxidized and colorless when reduced, thus forming a complement with the first electrochromic layer. The oxidation and reduction of an electrochromic counterelectrode also occurs by electron injection and insertion of the same charge compensating ions as for the first electrochromic layer. The charge compensating ions are transported by an ion conducting layer, e.g., an electrolyte, that is electron blocking and separates the two electrodes.
The operation of an electrochromic device in which the primary electrochromic layer is tungsten trioxide (WO.sub.3), the counterelectrode is vanadium pentoxide (V.sub.2 O.sub.5), and lithium ions (Li.sup.+) are transported is illustrated in accordance with reaction (1), below. ##STR1##
"x" is a stoichiometric parameter greater than "0" and less than "0.5"
Coloration is accomplished by the transfer of lithium (Li) from V.sub.2 O.sub.5 to WO.sub.3. Assuming that V.sub.2 O.sub.5 is the limiting electrode and that sufficient WO.sub.3 is present to accept all of the available lithium (Li), the dynamic optical range of the device will be determined by the amount of Li present.
The inserting species, for example lithium is introduced during fabrication of the electrochromic device. This can be accomplished by several methods, including the direct vapor deposition of the counterelectrode with the inserting species, for example, Li.sub.x V.sub.2 O.sub.5, the chemical reduction of the counterelectrode by elemental lithium in a separate processing step, or by electrochemical reduction in an electrolytic solution of lithium ions (Li.sup.+).
Alternatively, the lithium may be introduced initially into the primary electrochromic layer, for example Li.sub.x WO.sub.3. Or the lithium may be distributed between both electrodes, with the resulting structure in an intermediate state of coloration.
In a large area film processing, vapor deposition is widely used, so that the multiple layers of the structure can be deposited in-line on a continuous basis, as by using a series of vapor box coaters. For even coating of relatively large glass or plastic surfaces, in architectural and vehicular applications, sputtering is presently an industry standard.
In order to fabricate an electrochromic device by vacuum processing, such as by sputtering, the layers are deposited in Sequence, one on top of the other. For the high sputter rates needed for economical processing of large areas, sputtering sources with a good electrical conductivity are preferred.
Since the component layers of electrochromic devices are oxides, they are generally deposited in an oxidizing atmosphere, such as in pure oxygen O.sub.2, a mixture of argon and oxygen Ar/O.sub.2, or a mixture of oxygen and moisture O/H.sub.2 O. Consequently, the layers are reactively sputtered from targets of the parent metals. On the other hand, the insertion atom, such as lithium, must be deposited in a reducing or inert atmosphere. The resulting compounds, Li.sub.x V.sub.2 O.sub.5 and/or Li.sub.x WO.sub.3, for example, are easily oxidizable.
In subsequently fabricating an oxide ion conducting layer on top of a reduced electrochromic layer, such as Li+conducting glass of lithium silicate Li.sub.4 SiO.sub.4 or lithium-alumino silicate LiA1SiO.sub.4, it is necessary to have exposure to an oxidizing atmosphere, with potentially adverse effects.
In some cases an attempt to avoid the adverse effects has been made by converting WO.sub.3 to Li.sub.x WO.sub.3 by vacuum deposition, then capping the converted material with an ion conductor.
It is an object of the invention to provide for reducing an electrochromic layer, and then adding an ion conducting oxide without oxidizing the electrochromic layer. These two acts are accomplished concomitantly. Subsequent layers may then be deposited without significantly oxidizing the reduced electrochromic underlayer.
A further object is to avoid depletion of inserted ions in electrochromic layers during subsequent processing steps
Another object of the invention is to eliminate the need for a "capping" layer on the reduced electrochromic layer in order to avoid any subsequent oxidation of the electrochromic layer.. A related object is to eliminate any reduction in ionic conductivity associated with the use of a capping layer.
Yet another object is to avoid the occurrence of spontaneous coloration during the ion-conducting layer formation which tends to deplete the ion-conducting layer of insertion atoms. The result of such depletion can reduce the ionic conductivity of the ion conducting layer or possibly even form an ion-blocking layer at the interface between the electrochromic electode layer and the ion conducting layer, or between the interface of the counter-electrode layer and the ion-conducting layer.
Still another object is to avoid the objections associated with the evaporation of Li.sub.3 N on WO.sub.3, giving rise initially to dry injection of the Li to form Li.sub.x WO.sub.3, with the evolution of nitrogen gas N.sub.2. A related object is to avoid the build-up of a lithium nitride Li.sub.3 N ion conductor on WO.sub.3.
A further object is to avoid the problems associated with Li.sub.3 N, including red coloration and ease of oxidation on deposition of contiguous counterelectrode and conductive oxide layers.